building 48v 10ah pack

[magicaffair]

Hi all, my gm battery pack (48v 12ah) has died, but it was old and had good use. I have purchased 30 x 3.2v 10ah lifepo4 batteries , with the intention of building them into a new pack for my Magic Pie. I plan to run 15 in series and make 2 packs... then try and source bms for the packs. Any help on the following would be really appreciated :-)
 1. sourcing correct bms for each pack... (can I use a 16s bms board or does it have to be a certain type?)  I've found couple in the uk, but does it need an exact specification?
 2. Installing the bms (wiring to bms etc... I'm decent with a solder gun)
 3. How can I charge the pack? Can I use generic chargers, or do I need a lifepo4 charger?
 4. best way to connect all this lot and run it safely    

any help or suggestions appreciated... I'm off to buy voltmeter and solder gun in prep 

[Bikemad]

The BMS will need to be suitable for LiFePO₄ to give the required protection, and the maximum and minimum voltages should be pre-set to suit your particular cells. The current output would also need to be suitable for your cells too.
So you wouldn't want a BMS with a continuous output of 50 Amps if your cells were rated at 2C discharge and could only manage 20 Amps continuous.

It should be possible to use a 16s bms board with 15 cells, but you should check with the supplier before purchasing them.

The BMS will have one or more a multi-connector for the balance wires that will need to be connected to each of the cell connections similar to this:
 


The charger leads are usually wired through the BMS too, but you would need to check the wiring diagram for the chosen BMS to see exactly where these are attached.

The battery charger for your old GM pack should be set @ 54.6V and should therefore be suitable for charging your 15 cell LiFePO₄ pack (which would typically charge to 54.75V ). The slight difference of 0.01V per cell is not worth worrying about.

If you only have one bike, it might be better to build a single 48V 20Ah pack instead (15s2p) which would give the following advantages:

• Only one BMS would be required instead of two
• Twice the available continuous and maximum current output
• 50% less load on the cells
• Less voltage sag under load (a higher voltage with the same current draw = more power)
• More than twice the range of a single 10Ah pack
• All cells would be charged simultaneously from your existing charger

Although, this would also mean a big increase in both the size and weight of the pack.

Alan
 

[magicaffair]

thanks for that Alan, will fire off an email to these guys and get spec. on the board. Could I use the bms from my dead pack? it was used to balance out my other older GM lithium pack so would be set up for 48v 12ah, and 13s3p? any way I could use this part? just wondering?

[Bikemad]

Unfortunately your old BMS is not suitable for use with LiFePO₄ cells because their working voltage range is much lower than the GM LiMn cells.

You could probably reuse the old BMS and case etc. if you rebuilt your old pack using lithium cells from laptop batteries, or perhaps you could modify the casing to accommodate 13 LiPo cells (or 26, 39 etc.).

Alan
 

[magicaffair]

I've tested my old 48v 12ah GM battery at the + and - terminals and it's reading 52.8v?? I'm guessing this means it isn't dead..?! the individual cell voltages must be decent enough still i'm guessing. I'm going to take the pack apart and test the individual cells at the bms board.
It's worth noting that the battery did take a knock shortly before it stopped working (charged until done, but would just get 1 red light on throttle when plugged in?)
Any suggestions to aid me with diagnosing this one would be appreciated.  Have a new 1000w motor/controller to hook up, as bike was stolen recently only left with battery...  Would like to fix the old battery and get some use!

[Bikemad]

Ideally you should re-check the battery voltage while it is under load to see how much it drops.
A big voltage drop would indicate either a poor connection (possibly the switch contacts) or one or more faulty cells.
If the voltage is so low that only one LED it lit on the throttle, it should be easy to locate the voltage drop within the battery by measuring the voltages in and out of the switch, and also on the balance lead connector for all the cells.

Alan